STKUCTUKE OF OVAKIAN OVUM 



STRUCTURE AND DEVELOPMENT OF THE OVUM. 



Structure of the ovum. The mature human ovum ready for fertilisation 

 and outside the Graafian follicle has not yet been observed. It is necessary, 

 therefore, to begin with a 

 stage prior to the formation 

 of the polar bodies, and 

 homologous with the sperma- 

 tocyte in the male series 

 the stage of the oocyte. 



The oocyte (ovarian 

 ovum) (fig. 15) resembles 

 that of all other mammals 

 (with the exception of mono- 

 tremes) in its minute size. 

 Immediately before the time 

 of its discharge from the 

 Graafian follicle of the ovary 

 in which it has been formed, 

 it is a small spherical vesicle 

 measuring about *22 to 



*32 mm. in diameter, and is 



FIG. 13. DIAGRAM OF SPERMATOGENESIS. (T. H. Bryce.) 



F, two cells of Sertoli attached to wall of tubule; Spg., 

 a spermatogonium ; Spc. I., spermatocyte of the first order ; 

 Spc. II., spermatocyte of the second order; Spd., spermatids. 



just visible as a clear speck 

 to the naked eye. When it 

 is examined with the micro- 

 scope, in a fresh condition in 



the liquor folliculi, it is found to be invested by a comparatively thick, clear 

 covering. This, when the centre of the ovum is exactly focussed, has the 



FIG. 14. DIAGRAM OF THE DEVELOPMENT OF THE SPEBMATID INTO THE SPERMATOZOON. (T. H. Bryce.) 



The cell-body is represented in 1 by the outer circle ; its gradual reduction and displacement till it 

 forms the middle piece with its spiral filament in 9 is shown. The nucleus is represented by the inner 

 circle ; it forms the main part of the head. The modified attraction- sphere in 1 and 2 shows a central 

 body and a vesicle (archoplasmic vesicle, Moore and Walker). The central body (shaded) forms the cap ; 

 the vesicle becomes the tail-sheath (Moore and Walker). The centrosomes are shown as dots; the 

 growth from one of them of the tail-filament is seen. 



appearance in optical section of a clear girdle or zone encircling the ovum 

 (fig. 15), and was hence named zona pellucida by von Baer (1827). Under a 



